This invention relates to chelated, acrylic-based pressure sensitive adhesives which may be applied as hot melts. It also relates to an improved process for preparing these adhesives by controlling, not only the degree of reversible crosslinking via the amount of chelating conomoner used, but also via partial or complete chelation. It further relates to the use of the chelated pressure sensitive adhesives to form coated articles.
As used herein, the term "chelated copolymer" refers to a higher molecular weight, cohesive copolymer formed when a low molecular weight, normally tacky random copolymer containing a chelating comonomer is reacted with a chelatable transition metal salt containing a metal ion having a valence greater than 1 and a coordination number greater than 2. Interaction between the polymer chains, via the formation of thermally reversible chelate rings (chelates), causes crosslinking. When less than the maximum number of chelate rings are formed, the copolymer is referred to as "partially chelated copolymer". The term "chelating comonomer" refers to a monomer having reactive sites located in such positions that they can form a stable chelate ring incorporating the metal ion.
Pressure sensitive adhesives are viscoelastic materials which in solvent-free form remain permanently and aggressively tacky and which will adhere instantaneously to most solid surfaces with the application of very slight pressure. Hot melt adhesives are applied in a molten state and form a bond on cooling to the solid state.
Acrylic-based polymers have found extensive use as the base polymers for water-based and solvent-based pressure sensitive adhesive compositions because of their clarity, adhesion to non-primed surfaces, good aging characteristics, as well as excellent properties of tack, peel, and cohesive strength. Attempts to utilize these acrylic-based polymers in pressure sensitive, hot melt adhesives have not generally proven successful, however, because of the difficulties associated with the required melt viscosity. In order to have a hot melt adhesive that is sufficiently fluid at application temperatures, either a very low molecular weight polymer must be used or a high molecular weight polymer must be extended with a high proportion of low molecular weight oils or resins (tackifiers) to reduce the melt viscosity. Both of these alternatives are undesirable. The low molecular weight polymers will produce adhesives which lack cohesive strength unless subjected to an additional crosslinking step subsequent to application, such as exposure to ultraviolet radiation or electron beam radiation (see, respectively, U.S. Pat. Nos. 4,052,527 and 4,234,662 issued Oct. 4, 1977 and Nov. 18, 1980 to S. D. Pastor et al.). The use of high molecular weight polymers necessitates the use of expensive additives which frequently detract from the cohesion, adhesion, or tack of the resultant adhesive. In the past, it has often been necessary to compromise certain properties of these hot melts in order to obtain a workable adhesive having a very narrow range of utility.
Attempts have been made to produce acrylic-based hot melt adhesives exhibiting pressure sensitive properties by incorporating, in copolymer systems which have reactive sites supplied by comonomers, salts which crosslink with the reactive sites, thus producing an adhesive with improved cohesive strength. Such attempts have included crosslinking copolymers containing hydroxyl groups by reaction with metal alkoxides (e.g. tetrabutyl titanate) as in U.S. Pat. No. 3,532,708 issued Oct. 6, 1970 to R. B. Blance and polyvalent metallic salts (e.g. zinc acetate) as in Brit. Pat. No. 1,259,459 published Jan. 5, 1972 to A. M. Citrone et al.; those containing carboxyl groups, optionally substituted with ammonium or amine groups, by reaction with polyvalent metallic salts (e.g. zinc and cadmium acetate, glycinate, or glycollate) as in U.S. Pat. No. 3,740,366 issued June 19, 1973 to F. T. Sanderson; and those containing hydroxyl, carboxyl, or enolizable keto groups by reaction with esters of orthotitanic acid (e.g. dialkoxytitanium diacetylacetonate) as in U.S. Pat. No. 3,886,126 issued May 27, 1975 to L. W. McKenna, Jr. Hydrated aluminum nitrate and metal alkoxides (e.g. titanium acetylacetonate) have also been used to crosslink hydroxyl- or carboxyl-containing copolymers as described, respectively, in U.S. Pat. No. 3,677,985 issued July 18, 1972 to J. Sirota et al. and U.S. Pat. No. 3,769,254 issued Oct. 30, 1973 to C. C. Anderson et al.
However, these crosslinked acrylic-based pressure sensitive adhesive formulations have not been successful as hot melts either because the crosslinks formed by the reaction with the metallic salts are not broken or dissociated at the temperatures normally encountered during hot melt application (i.e. 70.degree.-175.degree. C.) or because the crosslinks are irreversible, as is the case with carboxyl-containing copolymers, and will not reform after being broken.
Hence, there is a need in the art for pressure sensitive, hot melt adhesive formulations which possess the reversible properties of strong cohesive strength at ambient temperatures, as well as desirable melt viscosity at the application temperatures used for hot melts.
One such acrylic-based pressure sensitive, hot melt adhesive formulation is described in U.S. Pat. No. 3,925,282 issued December 9, 1975 to I. Davis et al. The copolymers contain tertiary amine-containing monomers (e.g. diethylaminoethyl methacrylate or 4-vinyl pyridine) which provide the reactive sites. Reversible crosslinks are formed by coordination of an organic metallic salt (e.g. cupric laurate, zinc resinate, cadmium octoate, etc.) with the reactive amine site which donates electrons to the electron deficient metal ion.
It is an object of this invention to provide pressure sensitive, hot melt adhesives based on the reaction product of acrylic-based copolymers containing a chelating comonomer reacted with selected chelatable metallic salts.